The need for alternatives to fossil fuels for power production using an indirect (i.e., binary cycle) system is well known; and geothermal resources represent a promising solution. Geothermal fields are known that produce two-phase geothermal fluid in the form of high pressure steam and hot brine. In some fields, the underground pressure of this fluid is high enough to effect the passage of fluid from a source deep in the ground to the surface.
The usual approach to harnessing this fluid for power production using an indirect (binary cycle) system is to apply the geothermal fluid to a separator that separates the fluid into a steam component and a hot brine component. Heat is extracted from each component, and the heat depleted fluid is then returned to the ground though an re-injection well.
In one type of installation, the steam component is passed through an indirect heat exchanger containing an organic liquid that has been preheated by the brine component causing the organic liquid to be vaporized. The heat depleted steam component, and the heat depleted brine component are then combined and injected into the ground so as to prevent escape into the environment of any deleterious gases in the geothermal fluid. This type of installation is disclosed in U.S. Pat. No. 5,038,567 and in copending application Ser. No. 07/952,156, filed Sep. 28, 1992 (which is a continuation of application Ser. No 07/658,303 filed Feb. 20, 1991, now abandoned, the disclosures of all of which are hereby incorporated by reference.
The vaporized organic fluid is applied to an organic vapor turbine wherein expansion takes place causing a generator coupled to the turbine to generate power, and producing expanded, organic vapor. A condenser condenses the expanded organic vapor, and a pump returns the condensed organic fluid to the heat exchanger completing the cycle.
In installations like this, an inconvenience arises when the geothermal fluid contains a great deal of non-condensable gases. Provision must be made to extract these gases from the heat exchanger in which steam condensation takes place, namely the heat exchanger that acts as a vaporizer for the organic fluid. Failure to remove non-condensables on a regular basis will result in decreased heat transfer efficiency and an eventual halt in operations.
Conventionally, the problem is solved by extracting the non-condensables, pressurizing them, and optionally returning them to the ground with the heat depleted steam and brine. However, a considerable amount of power is required for this operation thus reducing the net power available from the power plant installation.
In addition, one of the limitations of producing power from two-phase geothermal fluid is the value of the temperature to which the geothermal fluid can be cooled. As the temperature to which the geothermal fluid can be cooled decreases, and the power output increases, some dissolved solids (such as silica) may precipitate. In the conventional approach described above, separating the steam from the brine concentrates the remaining brine and changes its pH further aggravating the problem of precipitation in the separated brine.
It is therefore an object of the present invention to provide a new and improved method and apparatus for producing power from two-phase geothermal fluid containing a substantial (e.g., above about 3%) amount of non-condensable gases, which produces more power and is simpler to construct and to maintain.